Organs may only be removed from a potential organ donor when death has been definitely determined. Until the mid-twentieth century, the standstill of respiration and heartbeat was the predominant criterion of death (cardiac death). However, patients with cardiovascular failure may sometimes be reanimated by means of modern methods of intensive care, and can recover. It hence became necessary to establish an additional criterion for the determination of death. The Ad-Hoc Committee of the Harvard Medical School to Examine the Definition of Brain Death took the lead in the establishment of such a criterion. The results of the Committee’s work were published in the so-called Harvard Report in 1968. According to the definition of the German Medical Association (Bundesärztekammer – BÄK), brain death is a state of irreversible cessation of the total function of the cerebrum (upper brain), the cerebellum (little brain) and the brain stem while the cardiovascular function is artificially maintained by controlled ventilation. The cerebral functions are irreversibly lost when the brain lacks blood and oxygen supply for a few (at most 10) minutes. After this time, the brain is cut off from the blood circulation and its cells degenerate and die, even if the rest of the body is still being supplied with blood by means of artificially maintained ventilation and cardiac action. Hence it is possible to remove still intact organs for transplantation purposes from a brain-dead patient. The BÄK has established Guidelines for the determination of brain death. Pursuant to these Guidelines, brain death must be diagnosed beyond doubt by two independently acting physicians who are not part of the transplantation team and who have several years of professional experience in intensive care medicine or neurology. In addition, the cessation of all reflexes of the brain stem and of spontaneous breathing must be demonstrated in a clinical examination. While brain stem reflexes can still be triggered in unconscious patients, they are absent in brain-dead patients. These reflexes include:

• Pupillary reflex: In healthy persons, both pupils are normally equally wide; they narrow when exposed to light. Brain-dead patients lack this reflex; their pupils are no longer reactive to light.
• Doll's eye reflex (oculocephalic reflex): If a patient is unconscious but not brain-dead they react to brisk turning or tipping of their head with slow eye movement in the opposite direction. The eyes of a brain dead patient however do not react to this test and remain in their initial position.
• Corneal reflex: When the outer layer of the eye (cornea) comes in contact with a foreign object, the eyes close as an automatic reflex. When the physician tests this reaction by touching the cornea of a brain-dead patient with a cotton swab, this reflex is absent.
• Response to pain in the face: Even patients who are in deep coma respond to painful stimuli that are applied to the face with distinguishable twitching of the muscles and defence reactions of the head and neck muscles. Brain-dead patients lack these reflexes.
• Gag- and cough reflex (tracheal and pharyngeal reflex): Touching the back of the pharynx induces a gag reflex in healthy and unconscious persons. This reflex is absent in brain-dead patients.

If all five reflex tests are suggestive of brain death, the physician will perform a spontaneous breathing trial. Automatic breathing is a vital reflex. When mechanical ventilation is stopped the carbon dioxide level in the blood increases due to the consumption of oxygen. This immediately activates the respiratory centre in the brain, which triggers a breath. The absence of spontaneous breathing is characteristic of the complete failure of the respiratory centre.

The last step is the determination of irreversibility of brain damage. An instrumental diagnosis of such damage is only mandatory in children under the age of two in case of primary damage of the posterior cranial fossa. In other cases a minimum monitoring period of 12 or 72 hours (depending on the kind of brain damage) is sufficient.

Research shows that the diagnosis of brain death varies across countries. While instrumental diagnostics is generally required in several states (like Norway, Luxemburg, France, the Netherlands, Mexico and Argentina), in Germany it is only mandatory in the aforementioned cases. Furthermore, the thresholds of these diagnostic tests do differ.